Glass-reinforced polymer (GRP) composites are widely used electrical insulators. GRP composite materials gradually absorb moisture over long periods of time when placed in a moist environment. The resulting leakage current can significantly impair the material's performance as an electrical insulator. In particular, there has previously been no practical way to predict the long-term electrical insulation properties of GRP composite materials in a moist environment.
In the past, the Applicants have performed high-voltage diffusion experiments utilizing a combination of the controlled moisture diffusion experiments and dielectric testing pursuant to ANSI Standard C29.11 Section 7.4.2 to evaluate the response of various glass reinforced polymer composites to moisture, and its effect on leakage current. For example, the tests were performed on solid composite rods by submerging them in boiling water and 0.1% NaCl solution for 100 hours and then measuring leakage currents. Despite the fact that very useful information was obtained about relationships between absorbed moisture and leakage currents in various unidirectional GRP composites with different surface conditions, no correlation was found between the mass gain and the leakage currents developed in the composites. Also, no attempt was made to correlate the rates of moisture absorption to the rates of increase in leakage current.
In another standard by ASTM D5229/D5229M-92 moisture absorption of materials can be measured using plates. Results for different materials can then be compared to either a Fickian single-phase model or non-Fickian double or multiple phase models. Also, a model was proposed by Carter and Kibler of anomalous diffusion that can be applied to handle non-Fickian diffusion (H. G. Carter and G. Kibler, “Langmuir-Type Model for Anomalous Moisture Diffusion in Composite Resins,” Journal of Composite Materials, vol. 12, pp. 118-131, 1978).
In contrast to the prior art in this field, the present invention shows a linear relationship between moisture contents and changes in leakage currents. In addition, the thin-walled specimen geometry used in the present invention allows large moisture concentrations to be absorbed in short periods of time by different classes of dielectrics. This methodology allows measurement of leakage currents corresponding to different amounts of moisture contents absorbed by different classes of dielectrics. This methodology can also be used to predict the maximum moisture contents and maximum leakage currents in various classes of dielectrics absorbing moisture according to double-phase diffusion based on the Carter and Kibler model.